Thermodynamics THERMODYNAMIC SYSTEMS AND PROCESSESoriginal conditions if the process was reversed. For example, an automobile engine does not giveback the fuel it took to drive up a hill as it coasts back down the hill.There are many factors that make a process irreversible. Four of the most common causes ofirreversibility are friction, unrestrained expansion of a fluid, heat transfer through a finitetemperature difference, and mixing of two different substances. These factors are present in real,irreversible processes and prevent these processes from being reversible.AdiabaticProcessAn adiabatic process is one in which there is no heat transfer into or out of the system. Thesystem can be considered to be perfectly insulated.IsentropicProcessAn isentropic process is one in which the entropy of the fluid remains constant. This will be trueif the process the system goes through is reversible and adiabatic. An isentropic process can alsobe called a constant entropy process.PolytropicProcessWhen a gas undergoes a reversible process in which there is heat transfer, the process frequentlytakes place in such a manner that a plot of the Log P (pressure) vs. Log V (volume) is a straightline. Or stated in equation form PVn = a constant. This type of process is called a polytropicprocess. An example of a polytropic process is the expansion of the combustion gasses in thecylinder of a water-cooled reciprocating engine.ThrottlingProcessA throttling process is defined as a process in which there is no change in enthalpy from stateone to state two, h1 = h2; no work is done, W = 0; and the process is adiabatic, Q = 0. To betterunderstand the theory of the ideal throttling process let’s compare what we can observe with theabove theoretical assumptions.An example of a throttling process is an ideal gas flowing through a valve in midposition. Fromexperience we can observe that: Pin > Pout, velin < velout (where P = pressure and vel = velocity).These observations confirm the theory that hin= hout. Remember h = u + Pv (v = specificvolume), so if pressure decreases then specific volume must increase if enthalpy is to remainconstant (assuming u is constant). Because mass flow is constant, the change in specific volumeis observed as an increase in gas velocity, and this is verified by our observations.Rev. 0 Page 29 HT-01
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